1. Technical Field
The present invention involves the guidance and positioning of a magnetic tape to assure its travel in a precisely determined tape path past writing or reading transducers.
2. Description of the Related Art
A widely used medium for the storage of magnetically recorded data is the data cartridge. Data cartridges typically have an enclosure containing first and second tape reels, a length of tape wound onto the reels and extending between the two reels, and an opening through which a tape drive can apply one or more transducers to the tape while transporting it between the two reels, so as to record data onto the tape or read data from the tape. A typical mechanism for transporting the tape from one reel to the other includes the use of a transport belt to the outer peripheries of the reels of tape, as disclosed by U.S. Pat. No. 3,692,255 (Von Behren).
It is common practice to write digital data onto recording tapes in rows, or tracks, running in the longitudinal, or tape transport, direction. Improvements in magnetic recording media and in magnetic recording apparatus, particularly improvements in magnetic writing and reading transducers, or heads, have created a trend toward writing data onto tapes in ever narrower and more closely spaced tracks. This has placed increased demands on the apparatus used to find and follow the data tracks, since as data tracks are made narrower and more closely spaced, the increased level of precision required to position a read transducer relative to a data track requires that the path followed by the tape as it passes the head also be more precisely and reproducibly determined.
One arrangement for achieving increased precision in positioning a moving magnetic tape relative to a read or write head is disclosed by U.S. Pat. No. 4,534,523 (Zarr), wherein several guides and wrap pins are used to guide the tape past the head. Each guide and/or wrap pin contacts a major tape surface and has a shoulder which contacts an edge of the tape. The shoulders contact alternating edges of the tape, at longitudinally spaced apart locations, so that the tape is not contacted by two shoulders directly across from one another, on opposite edges, at any one point along the tape. As a result, the spacing between a line passing through any two adjacent edge guide shoulders on one edge of the tape and the edge contacting surface of the guide shoulder intermediate between these two guides on the opposite edge of the tape can be slightly less than the width of the tape, without crushing or otherwise damaging the tape, since the tape is able to negotiate the path between the guide shoulders by traveling in a slightly weaving, or what Zarr calls a "somewhat sinusoidal," path. It has been found that this arrangement for tape guidance, called alternate edge guidance, constrains the tape to travel over a highly reproducible path, while reducing the risk of edge damage to the tape.
Since the spacing between the line through any two edge guiding shoulders on one edge of the tape and the intermediate edge guiding shoulder on the opposite edge of the tape is preferably only slightly less than the width of the tape, the spacing between this line and the intermediate guide must be closely controlled. Zarr does this by press fitting a pin in the base plate of the cartridge, and providing the top edge guide shoulders in the form of split rings which are positioned a fixed distance above the base plate and then crimped or compressed onto the guide pins. The guide pins themselves, along with the bottom guide shoulders, can be produced from solid rod material by screw machine operations. Alternatively, the bottom guide shoulders can be washers or disks captured between the guide pin and the base plate.
While crimped split rings can provide an effective means for edge guidance of magnetic tapes, they suffer from certain limitations, among them difficulty of assembly and limited choice of suitable materials. Installing a split ring onto a guide pin and holding it in a precisely located position while crimping it is a difficult task. The crimping operation involves a significant amount of deformation of the ring, as well as perhaps other parts of the assembly, which can in turn cause movement of the surface of the ring contacting the edge of the tape, thereby increasing risk of variability in the spacing between the edge guiding shoulders between which the tape must pass. The net result is a problematic build-up of tolerances.
An additional complication in installing a split ring onto a tape guide pin is that the split ring must be properly oriented on the guide pin so that the split portion does not contact the edge of the tape, since the split portion may contain burrs or other deformations which could damage the edge of the tape.
A further limitation encountered in the use of crimpable split rings as edge guides is that only a relatively few materials exhibit a suitable combination of mechanical and other properties for use in such an application. In particular, materials used in split rings must be sufficiently malleable to be crimpable without excessive springback, while at the same time exhibiting adequate strength.
Disadvantages also occur in the use of guide pins and shoulders produced by screw machining methods. In particular, producing such parts by screw machine methods places limitations on the materials which can be used, due to the quantity of stock material consumed by screw machine operations. In addition, machining operations are time consuming, and therefore costly. Further, since magnetic tape is sensitive to the surface quality of any surfaces which it contacts, the screw machining operation must be closely controlled in order to assure adequate surface quality, in particular surface smoothness which is in a suitable range. A further disadvantage of tape guides produced by screw machine methods is that they are not readily adjustable after machining. Therefore, the level of precision achievable in an assembled data cartridge will be dependent upon the accumulation of errors in the various components making up the tape guiding assembly.
Alternate edge guidance can also be achieved with only two tape guides, as disclosed in Japanese published patent application JP-130123, wherein two tape guides are slightly offset, so that a straight line path between the two guides is slightly narrower than the width of tape being transported, thereby providing a slight interference. Tape guiding by this means suffers, however, from difficulty of manufacture, since the amount of offset is very small relative to the overall width of the tape, and the two guides must be parallel to within a very small degree of error. This necessitates exceedingly precise insertion and orientation of the guides.
Tape guidance by means of edge contacting guides can produce an additional problem, namely that of variations in tape tension transverse to the tape transport direction. The quality of writing and reading of magnetic tapes depends upon intimacy of contact between the tape and the head, which in turn depends upon the tape tension. It is therefore desirable that the tape tension be uniform across the tape, so as to assure that the tape contacts the head uniformly across its width. When a tape path is modified even slightly by the application of guiding forces applied to the tape edges, transverse variations in tape tension can result in writing or reading errors.
Not only is it desirable to guide recording tape past read and write transducers in a precisely reproducible manner, it is also desirable to precisely guide the tape as it is wound onto the reels, so as to superimpose each strand of tape precisely over the previous strand as it is wound onto the tape pack, thereby producing an evenly wound tape pack which is less likely to cause edge damage to the tape. Evenness of tape guiding may be enhanced by providing additional guides, such as wrap pins 20 and 24 in U.S. Pat. No. 4,534,523 (Zarr). While useful for providing additional tape guidance, these wrap pins add frictional load to the tape drive, as well as adding to the cost of the cartridge.
There is therefore a clear need for precision tape guide components which can be more easily and precisely installed, which present a smooth surface of easily controlled quality to the edges and major surfaces of the tape, and which can be economically produced from materials known to be preferred for tape contacting and guiding applications. Additionally, there is a need for reduced cross-tape variations in tape tension. Finally, there is a need to reduce the number of tape guides or wrap pins contacting the tape, so as to reduce manufacturing cost and frictional drag on the tape.